How do I remove all metadata from a photo on Mac?

What do you mean by "All" metadata? Each and every exif tag and IPTC tag? Or only sensitive data like, faces, locations, capture date and time?

If you do not need to preserve the full quality, you can strip all metadata by taking a screenshot of the photo. Or open the image in Preview to export a PDF, then convert the PDF back to JPEG and all metadata will be gone, including camera model etc.

GraphicConverter > File > Browse... [select folder] > Metadata > Remove selected Data... > [select]

Or:

exiftool -all= image.jpg

There's a free app in the App Store, EXIFPurge, that does it very nicely. You'll need to use it outside of the Photos library.

The latest iOS usually strips the metadata/EXIF when you share it using the Share Sheet or the Photos app. Personally, I use a third-party iOS app, ViewExif, to strip it off before I share it.

On the Mac, using Photos, you can EXPORT the images and you are given options as to what information you want included OR you can look for a third party application, too.

Try this free open source tool: ImageOptim

You can get it here: https://imageoptim.com/mac